Field of the Application
The present application belongs to the technical field of rare-earth permanent magnetic materials, and specifically relates to a method for preparing a rare-earth permanent magnetic material with grain boundary diffusion using a composite target by vapor deposition, in which the composite metal film is formed on the surface of neodymium-ferrum-boron (NdFeB) by vapor deposition, and medium-high temperature treatment and low temperature aging treatment are carried out, so as to improve the performance of the magnets.
Background of the Application
The application of neodymium-ferrum-boron (NdFeB) permanent magnetic materials increases every day in the fields, such as hybrid vehicle, wind power generation and the like. These applications require magnets working at high temperature for a long time, therefore, it is necessary that the magnets having higher coercive force (Hcj) are needed. However, with thinning of the size of the NdFeB magnets machined, the coercive force is obviously decreased. Nd in principal phase Nd2Fe14B of the magnet is replaced with a heavy rare-earth element such as Dy or Tb to form (Nd, Dy)2Fe14B and (Nd, Tb)2Fe14B of which the anisotropy is better than that of Nd2Fe14B, this is an effective method for improving the Hcj of the sintered NdFeB magnet. However, the heavy rare-earth element Dy or Tb as a resource is scarce and expensive; in addition, the magnetic moments of Nd and iron are arranged parallelly, and the magnetic moment of Dy and iron and the magnetic moment of Tb and iron are arranged anti-parallelly, which causes the decrease of the remanence Br and the maximum magnetic energy product (BH)max of the magnet. Thus, seeking a preparing and treating method for effectively improving the coercive force and slightly decreasing the remanence and magnetic energy product, has become a consensus in the research and manufacture fields of the NdFeB magnet, and a lot of teams have committed to such research.
Recently, many research teams or individuals have disclosed a variety of the treating technologies of grain boundary diffusion in which the rare-earth element is diffused into the inside of the matrix from the surface of the magnet. The treating technologies of grain boundary diffusion mainly adopts coating, deposition, plating, sputtering, sticking and the like to adhere metal powders (Dy, Tb, or other rare-earth elements) or compounds to the external surface of the magnet, and adopts heat treatment to diffuse the metal powders or compounds into the principal phase of the sintered magnet through grain boundary, which greatly affects the composition, microstructure and magnetic performance of the sintered NdFeB magnets. Among them, the main technologies are evaporation, sputtering and coating. Evaporation or sputtering technology is that Dy, Tb and other rare-earth elements are deposited on the surface of the sintered NdFeB magnet, and subsequently heat treatment and diffusion are conducted. Coating technology is that the rare-earth compounds such as fluoride or oxide powders are coated on the surface of the magnet, and then heating is conducted to diffuse. The permeated rare-earth elements can be optimally distributed along with the grain boundary and the surface area of the grain of principal phase by the aforementioned method, thus, the coercive force not only is improved, but also the usage amount of precious rare-earth is saved and the remanence and magnetic energy product are not reduced significantly. However, there still are some problems that need to be solved: (1) the method in which Dy or Tb is adhered to the surface of the sintered NdFeB magnet using sputtering is low in productivity, is too high in production cost, easily introduces the defects such as melting pits and the like, and is low in utilization of heavy rare-earth metals since a large amount of rare-earth metals is distributed in the furnace chamber during evaporation; (2) the disadvantage of the method in which the fluoride or oxide powder of Dy or Tb is adhered to the surface of the magnet and heating is conducted, or the disadvantage of the method in which magnet is filled into the mixed powders of fluoride or oxide powders of Dy or Tb and calcium hydride powders and heating is conducted is as follows: the procedure number is increased, the cost is high, in addition, for coating the surface with oxide or fluoride of rare-earth and heating to diffuse, there are problems of limiting the increase of the coercive force, making other elements enter into the magnet with diffusion and etc. After the NdFeB magnet is machined and the surface thereof is cleaned by water-washing, acid-washing and etc., the NdFeB magnet is in the state of being capable to conduct the surface treatment of ion plating such as nickel plating, aluminum plating and the like, and then fluoride or oxide powder are adhered to the surface of the NdFeB magnet and heating is conducted. The surface layer being composed of oxide or fluoride of Dy or Tb for displacing Nd is formed on the surface after heating. Though the operation for adhering fluoride or oxide powder is cheap, the process for removing the surface layer will also increase the cost of the magnet; (3) in addition, as Dy or Tb is expensive, the most efficient use of Dy or Tb is also a key problem in this kind of technology.
Vapor deposition is a process in which gas raw materials react on the surface of the solid substrate and are deposited to form a solid thin layer or a thin film. At present, the method for improving the coercive force performance of the magnets in which a composite metal made from the heavy rare-earth element Dy or Tb and other elements such as Cu, Nd, Al, Pr and the like is evaporated onto the surface of NdFeB using composite targets by vapor deposition and the grain boundary diffusion is generated, has not been reported.